Electron beam controller

ABSTRACT

A magnet applies a magnetic field of predetermined intensity and shape to a spent electron beam over an axial distance having a predetermined relationship to the frequency of operation of a utilization device which has extracted energy from the electron beam and at a predetermined axial distance after the beam has left the magnetic field of the utilization device and entered the beam expansion region and beam stabilization region. The stabilizing magnetic field is terminated abruptly before the electron beam enters a collector apparatus.

United States Patent 1 Kosmahl Oct. 9, 1973 v [22] Filed:

[ ELECTRON BEAM CONTROLLER [75] Inventor: Henry G. Kosmahl, Olmsted Falls,-

Ohio

[73] Assignee: The United States of America as represented by the Administrator of the National Aeronautics and Space Administration June 27, 1972 [21] Appl. No.: 266,832

[52] US. Cl. SIS/5.38, 3l5/5.35

51 int. Cl. H0lj 23/02 58 Field of Search 315/5.35, 5.38

[56] References Cited UNITED STATES PATENTS 3,450,930 6/1969 Lien 315/538 X 3,297,907 1/1967 La Rue et a1.. 315/538 X 2,853,641 9/1958 Webber 315/538 3,273,006 9/1966 Osepchuk BIS/5.38 X 3,175,120 3/1965 Wendt 315/538 2,305,884 12/1942 Litton 315/535 Primary Examiner-Rudolph V. Rolinec Assistant ExaminerSaxfield Chatmon, Jr. Att0meyJ. R. vManning et al.

[57] ABSTRACT A magnet applies a magnetic field of predetermined intensity and shape to a spent electron beam over an axial distance having a predetermined relationship to the frequency of operation of a utilization device which has extracted energy from the electron beam and at a predetermined axial distance after the beam has left the magnetic field of the utilization device and entered the beam expansion region and beam stabilization region. The stabilizing magnetic field is terminated abruptly before the electron beam enters a collector apparatus.

12 Claims, 2 Drawing Figures PAIENIEDUCT SIQYS I I 3,764,850

AXIAL DISTANCE FIG 2 1 ELECTRON BEAM CONTROLLER ORIGIN OF THE INVENTION The invention described herein was made by an employee of the United States Government and may be manufactured and used by or for the Government of the United STates for governmental purposes without the payment of any royalties thereon or therefor.

I BACKGROUND OF THE INVENTION This invention relates to electron beam devices and is directed more particularly to apparatus wherein energy is extracted from an electron beam before the electrons of the beam are captured by a collector apparatus.

Modern space communication system require the highest possible efficiency from transmitting devices not only to conserve the primary power but also to minimize the heat dissipation which may occur in space due to poor efficiencies. Aside from high internal efficiencies, called electronic efficiencies, depressed-type collectors offer a solution for achieving high overall efficiency in microwave tubes. It can be shown, however, that the realization of energy recovery of such collectors depends strictly and critically upon the degree of disorder in the spent beam. Theoretically, 100 percent efficiency should be possible in a well-designed collector operating in relationship to a point source electron ticularly in a depressed collector. Consequently, it is apparent that a substantial dilution of current densities down to a few A/cm must take place before the electron beam is injected into certain type collectors if a high efficiency is to be achieved. In addition to diluting the current densities, transverse velocity components should be eliminated or minimized.

OBJECTS AND SUMMARY OF THE INVENTION It is an object of the invention to provide a method and apparatus for minimizing the transverse velocity components in a spent electron beam.

It is another object of the invention to provide for a spent electron beam apparatus for refocusing the beam, which apparatus is easily adaptable to a micro-' wave electron tube.

Still another object of the invention is to provide a method and apparatus for refocusing a spent electron beam utilizing negligible power or no power at all.

Yet another object of the invention is to provide a method for refocusing a spent electron beam for injection into a collector at high efficiency without any complex monitoring or programming steps being required.

A still further object of the invention is to provide apparatus for refocusing a spent electron beam with a minimum of parts.

It is an additional object of the invention to provide a method for refocusing a spent electron beam which is applicable to solenoid, permanent magnet or permanent periodic magnet focused tubes.

In summary, the invention produces refocusing of a spent electron beam by minimizing transverse elctron velocities in the beam whereby the electrons having a multiplicity of axial velocities are sorted at high efficiency by collector electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is the upper half of a longitudinal crosssectional view of a microwave oscillator tube embodying the invention.

FIG. 2 is a graph whose abcissa is proportional in length to the length of the tube of FIG. 1 and which is a plot of magnetic field strength versus axial length of the tube.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1,it will be seen that a microwave tube embodying the invention may be divided generally into an interaction region 10, a refocusing region 11, a beam stabilization region 12 and a collector region 13. The foregoing four regions are defined by vertical lines 14, l5, l6, l7 and 18.

An electron beam 19 will, in accordance with the instant invention, have the general shape as shown in I FIG. I wherein the spent beam from which energy has been extracted in region 10 has a relatively constant diameter as indicated at 20 in the interaction region 10 of the tube, increasing radius as indicated at 21 in refocusing region 1 l, stabilization of the radial velocities as shown at 22 by limiting of the velocities and of the radius to a maximum average fixed value as indicated at 23 in the collector region 13. Focusing and refocusing, as used herein, indicate that the transverse velocities of electrons comprising an electron beam are reduced. Consequently, although an electron beam may be expanding, a limiting or decreasing of the amount of expansion will reduce the transverse velocity components of electrons in the beam. This causes the axial velocity of the electrons to increase, as may be shown by the well-known adiabatic expansion.

The interaction region 10 may include a solenoid comprising a current carrying winding 24 and a pole piece 25 which produce a magnetic field of constant value as indicated at 26 in FIG. 2. Downstream of the first solenoid is a second solenoid comprising current carrying windings 27, a rear pole piece 28 and a front pole piece 29. The second solenoid produces a constant magneticfield in stabilization region 12 as indicated at 30 in FIG. 2 and of much lower intensity than the magnetic field of the first solenoid.

In stabilization region 12 the transverse velocity components of the beam are minimum. Advantageously, the groups having slower axial velocities drift preferentially to the edge of the beam thus providing a natural velocity-position sorting which reduces the sorting which must be done subsequently in the collector region 13. This presorting increases the collector efficiency because less energy is expended by the collector in sorting.

It will be seen that the magnetic field in the refocusing region 11 of FIG. 1 decreases smoothly but not necessarily'linearly, as indicated at 31 in FIG. 2, from its intensity as produced by the first solenoid to its lower intensity as produced by the second solenoid. The ratio of the magnetic field as given by 26 to that indicated by 30 in FIG. 2 is determined by the degree of the required dilution of the current density in the spent electron beam. Since the average beam radius is inversely proportional to the local magnetic field, the dilution is approximately given by the factor (B IB F, where B, and B designate the values of the magnetic fields in the tube region and the stabilization region 12, respectively. Because the current density J, in the tube region is known from the tube design, and the current density J,; at the entrance into the collector should not exceed the amount of approximately l0A/cm we have for the ratio (B /B J /J A pole piece 32 of smaller diameter than the pole piece 29 may be disposed on the front of the pole piece 29 so as to produce a smooth, gradual change of the magnetic field to its stabilizing value. This smooth change is shown at 33 in FIG. 2.

In the collector region 13 the electrons comprising electron beam 19 are all collected and, as shown in 38 in FIG. 2, there is no magnetic field in this region except for a residual amount as shown at 39 after the magnetic field beings an abrupt decrease towards 0 as at 40. The collector mechanism of the collector region 13 may comprise apertured dished plates 34, 35, 36 and 37. However, it will be clear to those skilled in the art, that other suitable collectors of the prior art such as those used for microwave electron tubes may be used. To apply an increasingly negative voltage to each of the collectors 34 through 37 in a direction away from the electron source, the collectors are connected to a voltage divider 41 which is energized by suitable d-c source 42. The positive side of the d-c source 42 is grounded as at 43 as are the pole pieces 25, 28, 29 and 32. Numerals 44 through 49 indicate some of the paths which may be followed by various electrons in the electron beam as they travel toward the collector plates.

Referring again to FIG. 2, L,, the distance between vertical dash lines and 16, and L the distance between the vertical dash lines 16 and 17, represent the length of the refocusing region and the stabilization region, respectively. In accordance with the invention, L, has a length from about one cyclotron wavelength to about three cyclotron wavelengths while L has a length from about one-half cyclotron wavelength to about 1 cyclotron wavelength, where a cyclotron wavelength is defined as -rrIfi/(e/m) B], where i, e/m and B designate the average electron velocity, the electronic charge to mass ratio equal to 1.76Xl0 inch Coul/Kg and the local magnetic field B, respectively. It will be understood by those skilled in the art of electron beam dynamics in microwave amplifiers, that the optimum lengths L, and L can be determined exactly only through complex complications such as to produce least transverse velocity at the entrance into the collector. However, the present invention gives the magnitude of the required fields the range of suitable lengths L, and L and the method and shape of the refocusing fields.

It will, of course, be understood by those skilled in the art that permanent magnets may be used in place of the solenoids described in FIG. 1.

If the tube is of a type utilizing permanent magnets for focusing in the interaction region to minimize weight and power consumption, then the magnets for the stabilizing region will preferably be also of the permanent type. In this case one single permanent magnet can be constructed such as to produce the required profile of the refocusing fields by a proper placement of pole pieces 25, 29 and 28 and proper shaping of the magnet itself.

What is claimed is:

1. A method of minimizing transverse velocity components in an expanding spent electron beam comprising the steps of:

restricting the expansion of said beam within predetermined limits for an axial distance of'from about one cyclotron wavelength to about three cyclotron wavelengths to reduce the radial velocity of electrons in the beam;

then stabilizing said beam for an axial distance of from about one-half cyclotron wavelength to one cyclotron wavelengths; and

collecting said electron beam.

2. A method of conditioning a spent electron beam after it emerges from a first magnetic field of a utilization device comprising the steps of:

subjecting said electron beam to a gradually decreasing magnetic field which tapers to a stabilizing intensity, said decreasing magnetic field having an axial length of from about one cyclotron wavelength to about three cyclotron wavelengths, subjecting said beam to said stabilizing intensity for an axial length of from about one-half wavelength to one cyclotron wavelength after which it becomes zero; and

collecting the electrons comprising said electron beam downstream of the point at which the stabilizing intensity drops to zero.

3. The method of claim 2 wherein said stabilizing intensity is from about V4 to l/IO as great as the intensity of said first magnetic field.

4. Apparatus for focusing an electron beam comprising:

means for limiting the expansion of said electron beam within a predetermined limit for an axial distance from about one cyclotron wavelength to about three cyclotron wavelengths:

means for substantially eliminating further expansion of said electron beam for an axial distance of from about one-half cyclotron wavelength to one cyclotron wavelength; and

means for collecting the electrons of which the beam is formed, each of said means being positioned in order as named in a direction away from said source of electrons.

5. An improved microwave tube comprising means for producing a beam of electrons;

means for extracting energy from said beam;

first magnetic means for focusing said beam;

second magnetic means disposed downstream of said first magnetic means to provide a constant magnetic field of lower intensity than that of said first magnetic means;

the fields of said first and second magnetic means merging to provide a refocusing field which decreases gradually in a downstream direction from the intensity produced by the first magnetic means to the intensity produced by the second magnetic means; and

means for collecting the electrons of said beam downstream of said second magnetic means.

6. The apparatus of claim 5 wherein said first and second magnetic means are solenoids.

7. The apparatus of claim 6 wherein said second solenoid includes a front pole piece including an upstream extending annular step portion of smaller diameter than said pole piece.

8. The apparatus of claim 5 wherein said first and second magnetic means are permanent magnets.

9. The apparatus of claim 5 wherein said collector is of the depressed type.

10. The apparatus of claim 5 wherein said second magnetic means includes auxiliary magnetic means disposed at the downstream surface of said second magnetic means to provide a smooth gradual change as the decreasing magnetic field between said first and second magnetic means merges into said lower intensity field produced by said second magnetic means.

11. The apparatus of claim 5 wherein said second magnetic means produces a magnetic field having an intensity 1/10 to /4 as great as the intensity produced by said first magnetic means.

12. The apparatus of claim 5 and including means for abruptly terminating the field produced by said second magnetic means upstream of said collecting means.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION- Patent No. 3,764,850 Dated October 9, 1973 Inventofls) Henry G. Kosmahl It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 3, line 48, change "1 7H'fi7(e/m) B] to i IT'[ line 55, change "comp liea'tifons" to -computations-- line 58, aZ'EtBge-"fieIGS" insert and and sealed this 30th day or April 197M.

(SEAL) Attest:

c MAR SHALL :DA-NN Commissioner" of Patents EDWARD M .FLE'I CHER JR Attesting Officer O M P9-1psq (10-69) 

1. A method of minimizing transverse velocity components in an expanding spent electron beam comprising the steps of: restricting the expansion of said beam within predetermined limits for an axial distance of from about one cyclotron wavelength to about three cyclotron wavelengths to reduce the radial velocity of electrons in the beam; then stabilizing said beam for an axial distance of from about one-half cyclotron wavelength to one cyclotron wAvelengths; and collecting said electron beam.
 2. A method of conditioning a spent electron beam after it emerges from a first magnetic field of a utilization device comprising the steps of: subjecting said electron beam to a gradually decreasing magnetic field which tapers to a stabilizing intensity, said decreasing magnetic field having an axial length of from about one cyclotron wavelength to about three cyclotron wavelengths, subjecting said beam to said stabilizing intensity for an axial length of from about one-half wavelength to one cyclotron wavelength after which it becomes zero; and collecting the electrons comprising said electron beam downstream of the point at which the stabilizing intensity drops to zero.
 3. The method of claim 2 wherein said stabilizing intensity is from about 1/4 to 1/10 as great as the intensity of said first magnetic field.
 4. Apparatus for focusing an electron beam comprising: means for limiting the expansion of said electron beam within a predetermined limit for an axial distance from about one cyclotron wavelength to about three cyclotron wavelengths: means for substantially eliminating further expansion of said electron beam for an axial distance of from about one-half cyclotron wavelength to one cyclotron wavelength; and means for collecting the electrons of which the beam is formed, each of said means being positioned in order as named in a direction away from said source of electrons.
 5. An improved microwave tube comprising means for producing a beam of electrons; means for extracting energy from said beam; first magnetic means for focusing said beam; second magnetic means disposed downstream of said first magnetic means to provide a constant magnetic field of lower intensity than that of said first magnetic means; the fields of said first and second magnetic means merging to provide a refocusing field which decreases gradually in a downstream direction from the intensity produced by the first magnetic means to the intensity produced by the second magnetic means; and means for collecting the electrons of said beam downstream of said second magnetic means.
 6. The apparatus of claim 5 wherein said first and second magnetic means are solenoids.
 7. The apparatus of claim 6 wherein said second solenoid includes a front pole piece including an upstream extending annular step portion of smaller diameter than said pole piece.
 8. The apparatus of claim 5 wherein said first and second magnetic means are permanent magnets.
 9. The apparatus of claim 5 wherein said collector is of the depressed type.
 10. The apparatus of claim 5 wherein said second magnetic means includes auxiliary magnetic means disposed at the downstream surface of said second magnetic means to provide a smooth gradual change as the decreasing magnetic field between said first and second magnetic means merges into said lower intensity field produced by said second magnetic means.
 11. The apparatus of claim 5 wherein said second magnetic means produces a magnetic field having an intensity 1/10 to 1/4 as great as the intensity produced by said first magnetic means.
 12. The apparatus of claim 5 and including means for abruptly terminating the field produced by said second magnetic means upstream of said collecting means. 